This invention relates in general to proportional poppet valve control and, in particular, to a self-adaptation scheme for poppet valves in an electrohydraulic brake system. Poppet valves can be used in electrohydraulic brake systems to control the pressure of brake fluid applied to vehicle wheel brakes.
Traditionally, proportional poppet valves are operated in a manner proportional to the voltage applied to the valve""s controlling solenoid. The valves are either closed, opened, or are in some intermediate position. Normally this prevents or allows a fluid to pass from one side of the valve to the other. As it relates to this invention, valves either allow or prevent hydraulic fluid flow in a hydraulic circuit, specifically in a hydraulic circuit in a brake system. An electrohydraulic braking (EHB) system utilizes electronically controlled valves, pumps, or a combination thereof to augment, replace, or control the base braking operation of a vehicle brake system. Base braking, sometimes referred to as foundation braking, is the basic braking called for by the operator of a vehicle. In base braking, the brake pedal operates a master cylinder, causing the master cylinder to send pressurized hydraulic brake fluid to the wheel brakes of a vehicle. Advanced braking systems, such as EHB systems, have been used to improve the performance of vehicle braking systems by augmenting or replacing the base braking function with other braking operations.
One of the first of many advanced braking functions that has been developed for vehicles was an Antilock Braking System (ABS), which typically involves the operation of valves and pumps to selectively release and re-apply brakes during a braking operation. While typical base braking is commanded by the operator, ABS braking controls the vehicle brakes to recover from and limit skidding of a vehicle""s wheels due to braking the wheels harder than permitted by the available coefficient of friction of the road surface. Since pumps and valves are electronically controlled to augment the base braking operation, a vehicle equipped with ABS may generally be said to have an EHB system.
Another advanced braking function that may be accomplished by a properly configured EHB system is VSC (Vehicle Stability Control), which is a system for selectively actuating vehicle brakes to improve the stability of a vehicle during vehicle maneuvers. Other braking applications producing a pressure command input to the present invention include DRP (Dynamic Rear Proportioningxe2x80x94a system for controlling the front to rear proportioning of a vehicle braking command), TC (Traction Controlxe2x80x94which typically involves selective application of brakes during vehicle acceleration to recover from and limit skidding of a vehicle""s wheels due to accelerating the wheels faster than permitted by the available coefficient of friction of the road surface), ACC (Autonomous Cruise Controlxe2x80x94a cruise control system that can actuate vehicle brakes to maintain proper vehicle spacing relative to a vehicle in front) and similar finctions.
A subset of electrohydraulic braking systems is electronic brake management (EBM). EHB systems can allow braking to be primarily controlled by the vehicle driver with a conventional master cylinder system. Additionally, an electronically controlled portion of the system operates the brakes under certain conditions, i.e. anti-lock, traction control, etc. In Electronic Brake Management systems, primary braking is controlled electronically. In an EBM system, the vehicle driver or a safety system generates an electronic signal, which in turn operates the pumps and valves to achieve a braking pressure within the system. A pedal simulator creates the effect for the driver of applying direct braking pressure while also providing a back-up braking system in case of a failure of the primary system. In the back-up system, the pedal simulator acts as a master cylinder during the failure event and provides the hydraulic pressure that actuates the brakes.
Regardless of the type of electrohydraulic braking system that is used, a system with proportional poppet valves has a control process that controls whether the valves are opened, closed or intermediately positioned. In order for a control system to properly control the poppet valves, it must be configured to account for the forces acting on the valves, the natural characteristics of the valves and be able respond to changes in the valve during braking operations.
Proportional poppet valves used in the above-described systems typically comprise a valve armature, a valve seat, and a spring. If an electric current controls the valve, there generally is a solenoid that acts upon a magnet causing the valve armature to be raised from or seated upon the valve seat. Because a valve can be normally open or normally closed, there are different forces acting upon the valve in its default position. Generally, such forces can be a magnetic force, a spring force, an inlet pressure force, or an outlet pressure force. The inlet and outlet pressure forces will ordinarily vary depending upon the load demanded within the system. In that voltage is proportional to current, it is understood that the use of current controls are to be within the scope of the claims of the present invention.
To balance the forces that are naturally occurring on the valve, so that a particular valve is either normally opened or normally closed, the closing boundary must be determined. A closing boundary also compensates for deadband in the system. Deadband compensation is used to reduce delays in valve response to an applied voltage when the valve is in its normal position. In proportional poppet valve pressure controls, a closing boundary is defined as the minimal (for normally open valves) or maximal (for normally closed valves) voltage required to keep an armature assembly in contact with the seat. Closing boundary data, as used for deadband compensation, gives minimal ramp lags but is dependent on how accurately the closing boundary is set. Several factors make it difficult to accurately set the closing boundary under all operating conditions. First, it is difficult to accurately measure the closing boundary. Second, the actual closing boundary changes in a time-variant manner due to operating pressure, temperature and potentially other naturally occurring phenomena. Lastly, the actual boundary varies from one valve to another because of manufacturing tolerances. Proportional poppet valve pressure control systems are very sensitive to inaccuracies in the closing boundary. This high sensitivity can be justified by the fact that poppet valve systems have a far smaller effective control band at a given pressure than systems with spool valves. Another difference between the two-valve system and the one-valve system is that the two-valve system utilizes an apply valve and a release valve to modulate the brake pressure at each wheel. Therefore, boundary variations in any one of the two valves can affect braking performance at a wheel.
One method that can be used to account for variations in multiple valves at one time is a lump-sum method. However, there is a limitation in using lump-sum estimation of boundary variation. The lump-sum approach generally achieves good pressure tracking performance but the system could converge to equilibrium where one valve is not closed while the other is supposed to be open. The degree of the problem in such a system varies with the amount of boundary variation. Another drawback is that given non-linearities in flow gain at a given pressure, the lump-sum effect of boundary variations in two valves would change rapidly whenever there is a change in the valve state (open and closed). As a result, every time there is a change in the valve state, transients are induced.
To exhibit ideal performance in a system, each valve would have to be trimmed individually to match the closing boundary. This individualized tailoring process is time-consuming and expensive to conduct for a mass-produced system. Therefore, it is important to devise a boundary self-adaptation scheme to produce the system in a cost-effective manner.
U.S. Pat. No. 6,086,167 to Heckmann, et al. describes a method and device for regulating wheel brake pressure. Pressure is regulated by a regulator generating a driving signal quantity for a pressure-influencing valve arrangement on the basis of the active operating point of the valve arrangement. Given a pressure differential across the valve arrangement, the operating point can be determined from a predetermined current-pressure characteristic curve. The characteristic curve essentially defines a point (at or near zero flow) from which up or down hydraulic flow is utilized to regulate wheel brake pressure. The boundary addressed in the present invention is the watershed between hydraulic bulk flow and leakage flow, both of which are utilized for wheel brake pressure control.
U.S. Pat. No. 6,030,055 to Schubert improves upon the quality of the pressure control system described in U.S. Pat. No. 6,086,167 and makes manual determination and adjustment of the characteristic curves unnecessary. Primarily, Schubert""s invention is based upon the alternative exemplary embodiment of U.S. Pat. No. 6,086,167, where a regulator based on pressure difference between reference pressure and actual wheel brake pressure outputs a pressure correction quantity to the reference pressure, and the corrected reference pressure in turn is used to find activation current from the current-pressure characteristic curve. Schubert describes a process that automatically equalizes the correlation between the pressure difference at a valve and the activation current. The correction quantity occurring in the course of a regulation operation is held within defined limits by appropriate adaptation of the characteristic curves. The limits of the correction quantity are determined as a function of the actual wheel brake pressure and the dynamic ratio of the reference pressure. The characteristic curve for the apply valve is modified during pressure buildup and the characteristic curve for the release valve is modified during pressure reduction.
An estimation approach that estimates boundary deviation should disregard performance changes due to other factors. A system that does disregard such other factors would be beneficial in achieving consistent and convergent estimation. Therefore, an estimation approach based on a different philosophy than that of the patents listed above would provide a more accurate response to a pressure command signal. Consistent estimation would in turn help generate consistent pressure control performance for different types of pressure commands.
This invention relates to a method for adapting a closing boundary for a proportional valve comprising implementing an estimator including an integral element to estimate boundary variations of the valve. This entails using the sign of the pressure command derivative over time to determine which one of the two estimators for a wheel should be updated. Next a modified pressure error is calculated in such a way that steady state pressure error, resulting from feedforward term mismatch, control deadzone, and other factors, is subtracted from measured pressure error. Finally, the modified pressure error is used as the input to the estimators and the boundary table is updated using the resultant boundary deviation estimates.
The proposed boundary self-adaptation scheme implements two estimators including an integral element that are used to estimate the boundary variations of two valves at each of several channels. Each channel can represent a wheel brake in a hydraulic circuit. The sign of the pressure command derivative over time is used to determine which estimator should be updated. When the pressure command derivative over time is positive, the apply estimator would be updated and the release estimator left unchanged. When the pressure command derivative over time is negative, the apply estimator is ignored and the release estimator is updated. A modified pressure error is calculated and is then used in the estimation. The steady state pressure error resulting from mismatched feedforward term, control deadzone and other factors are then subtracted from the measured pressure error. The only error that remains after completion of the above-described correction process, is the error due to boundary deviation. The boundary deviation error is then used to update the boundary table. Gain scheduling is used in the estimators to deal with potential asymmetry in mapping from boundary deviation to modified pressure error. To avoid transients and integral wind-up, the estimators are updated during gentle braking maneuvers. Finally, the whole pressure region is partitioned into segments with one state variable associated with each segment. The values of state variables are in turn used in computing control commands at the corresponding segments.
Various objects and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings.